Formation Evaluation Utilizing Gamma Ray Spectral Analysis

Abstract

ABSTRACT Since currently producing hydrocarbon reservoirs can no longer supply the world's energy needs, the search for new sources has led to another look at formations that have posed difficult evaluation problems. Several formations encountered in the Rocky Mountain region are in this category. In the past, drilling or evaluation techniques could not accurately indicate productive and nonproductive zones. Three of the stratigraphic units which have proven difficult to evaluate with conventional surveys are the Niobrara, Mawry and Phosphoria. Problems in evaluating these formations can result from several factors. Mast can be related to determining the true clay content for accurate lithology and resultant porosity determinations, the location of fractured or fracturable zones, and evaluating formations that contain a low or unknown salinity formation water. Recent developments in wireline logging instruments provide measurements which enable these and other sequences with similar problems to be correctly evaluated. These methods rely on the spectral analysis of natural and induced gamma rays. Two commercially available instruments that provide these measurements are the Spectralog and Continuous Carbon/Oxygen log. This paper will present the theory of operation and parameters measured with these instruments. Applications of these measurements will be discussed, with field examples showing how these measurements provide an improved evaluation of these formations.